The present invention relates generally to molded, plastic strain relief bushings. More particularly the present invention is related to plastic, two-piece, threaded strain relief bushings adapted to resist torsional deflections of the type seen in United States Class 285, Subclasses 161 and 343.
Conventional electric appliances including computers and their components require an electrical cord to pass through the case into the interior of the product. Typical devices generally include an orifice that receives a relief bushing to allow the cord to pass into the product without encountering a sharp edge. Such bushings are generally required by standardized codes. Typical strain relief bushings are used to secure insulated electrical cords against axial and torsional displacements to prevent damage and electrical shorts. In other words, these bushings prevent a tug or pull on the cord from tightening the cord inside the machine and thereby damaging or loosening the connection between the operative components of the product and the cord.
A wide variety of plastic strain relief bushings exist in the prior art. As will be appreciated by those skilled in the assembly of electrical and electronic equipment, a number of yet unsolved problems are encountered with the installation and use of known strain relief bushings.
In the prior art strain relief bushings of the two piece type are common. In many forms of prior art the two pieces are integrally attached to one another by some form of hinge-like strap structure so that both of the cooperating pieces are unitary. In other words, only one unitary piece exits from the mold during the manufacturing process, and only one part need be inventoried by the manufacturer or assembler. The two pieces are then folded together against yieldable tension from the connecting strap to appropriately align and insert the gripping block into the body portion about the electrical cord, for subsequent insertion into the previously established orifice in the work piece. Yet, two-part bushings often fail to provide appreciable structural strength.
U.S. Pat. Nos. 3,298,840 and 3,290,430 exemplify this structure. In these strain relief bushings the strap structure is generally flat, and it angularly extends between two pieces having their axis opposed by at least 90 degrees.
U.S. Pat. No. 4,729,534 is owned by the same assignee as in this case and depicts a unitary two part bushing. The twin parts have similar exteriors but different interiors. One half comprises an internal cavity penetrated by an intruding tongue emanating from the other. These components cooperate to lock a cord between them when the unit is forced into a predefined, non-threaded orifice in a workpiece.
In the prior art all known strain relief bushings of the threaded type comprise a relatively large number of parts that secure a tubular section either by separate nuts or a shoulder on the threaded tube. The element of these bushings that actually contacts the cord, the grommet, is secured within an end of the tube by a separate nut or threaded bushing. In many forms of prior art the grommet is unitary in structure and must be slid over a free end of the cord. This grommet is easily lost during production if it cannot be secured to the rest of the assembly immediately. Furthermore, future repair requires that the entire cord be removed to obtain access to a free end. Many times such procedures are not practical. However threaded bushings known in the prior art provide a relatively rigid and structurally strong means to secure cords in electrical products.
Illustrative of such threaded bushings are U.S. Pat. No. 4,030,742 issued to Eidelberg on Jun. 21, 1977 and, Fidrych U.S. Pat. No. 4,030,741 also issued Jun. 21, 1977. Both these prior art patents disclose a threaded body having an integral nut or shoulder and multiple pieces intended to encompass, shield and secure a cord, cable or conduit.
Many applications requiring electrical cords must have a relatively strong interface at the cabinet of the product. Commonly heavy electrical devices are handled by a cord or a cord is accidentally yanked. Either situation causes much stress to be asserted on the cord where it passes through the device's cabinet.
One major drawback of well-known threaded strain relief bushings is that installation is cumbersome due to the large number of pieces, and is therefore time consuming. This disadvantage also introduces inventory problems. More pieces mean more storage area and more time spent tracking the parts.
Another significant disadvantage of the prior art is its absolute sizing. The threaded housing associated with conventional threaded relief bushings is formed to pass through and secure to a specific size wall or similar structure.
Also, well-known prior art strain relief bushings generally fail under unusual strain. Under tension of continued duration, the two parts of the grommets weaken, become distorted, and eventually pull away from each other and the insulative jacket of the electrical cord and out of locking engagement with the threaded housing.
Numerous prior art patents exist for threaded strain relief bushings employing two or more cooperative, interlocking parts. For example, Nicholson, U.S. Pat. No. 3,843,833, dated Oct. 22, 1974, which includes interior teeth or serrations adapted to grip the jacket of the electrical cord. Law U.S. Pat. No. 4,900,068 discloses a liquid tight connector employing a clip with fingers to grasp conduit. U.S. Pat. No. 4,738,636 issued to Bolante on Apr. 19, 1988 shows structure that grips a cord with tines. Jones teaches in U.S. Pat. No. 4,789,759 an assembly that is secured to a bulkhead having three concentric bores the outer one of which is threaded. Elgin, U.S. Pat. No. 4,549,038 discloses a cord grip employing a tubular housing with a shoulder and the interior of which has a decreasing circumference to compress a grommet around a cord as the grommet is forced deeper into the housing by an exterior that gland nut.
Lundeberg U.S. Pat. No. 4,150,250 shows a stain relief fitting which has a collapsible insert. This insert confines a cord once it is compressed by the device's external cap nut. U.S. Pat. No. 4,375,011 Issued to Grunau Feb. 22, 1983 discloses a bushing employing a screw nut with a conical interior that compresses a bushing around a cord. Michaels, U.S. Pat. No. 4,350,840 discloses a rubber grommet compressed by a nut with a conical interior. Guginsky discloses in U.S. Pat. No. 4,835,342 a strain relief bushing having two interior components and a threaded housing with a decreasing diameter.
It would be desirable to provide a strain relief bushing that securely, nondestructively engages the insulative jacket of the electrical cord when the bushing is installed. Particularly, it would be advantageous to design an anti-twist strain relief grommet that is particularly adapted to non-destructively resist torsional movements. Such an anti-twist bushing must be capable of quick and easy installation, and a minimum of parts should be involved.